Backward Pass — Late Start (LS)
Late Start (LS) is the other key number that comes out of the backward pass — the trip we take through the schedule from the project's end date back toward the start to find the latest possible timing for each activity. LS answers the question: "What is the very latest this activity can begin and still not delay the finish of the whole project?"
To calculate it, you start with the activity's Late Finish (LF) — the latest it can finish, which you already found. Then you subtract the activity's Duration (how many days the work takes) and add 1. The "+1" is there because we are counting whole working days inclusively: if a task takes 3 days and finishes on day 9, it occupies days 7, 8, and 9 — so it must start on day 7, which is 9 − 3 + 1.
Reading it is straightforward: if the Late Start is much later than the Early Start (the earliest the activity could begin), the activity has flexibility. If LS equals ES, the activity has zero flexibility and sits on the "critical path" — the chain of activities that determines the project's finish date.
Project managers use LS to know the last safe moment to kick off a task. It's especially useful when juggling limited people or equipment, because it tells you how long you can safely wait before a task must begin.
Think of it like the latest time you can start baking a cake so it's ready for the party. If the cake must be done by 6 p.m. and it takes 2 hours, you can't start any later than 4 p.m. — that's your late start.
Suppose Activity A has a Late Finish (LF) of day 9 and takes 3 days to complete (Duration = 3). Its Late Start is: LS = 9 − 3 + 1 = 7. This means Activity A can begin as late as day 7 and still finish by day 9. If the crew starts on day 8 instead, they'd only have days 8 and 9 to do 3 days of work — impossible — so the project would fall behind.
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